The present invention relates to a disk-reproduced signal equalizer and, more particularly, to a disk-reproduced signal equalizer applied to a disk recording/reproducing apparatus of a CWL-CAV (Constant Wave Length-Constant Angular Velocity) scheme in which a recording/reproducing operation is performed at an almost constant recording wave length by changing the clock frequency in accordance with a track position while keeping the rotational speed of a disk constant.
In the recording/reproduction frequency characteristics of a disk recording/reproducing apparatus, the amplitude of a high-frequency component is generally smaller than that of a low-frequency component. For this reason, a reproduced signal equalizer is arranged on the output side of a reproduction head to emphasize the high-frequency component of a reproduced signal.
For example, data recording schemes for disk media include a CAV (Constant Angular Velocity) scheme of recording data at a constant bit rate while keeping the rotational speed of a disk constant, a CLV (Constant Linear Velocity) scheme of recording data at a constant bit rate while keeping the track linear velocity constant by changing the rotational speed of a disk in accordance with a track position, and a CWL-CAV (Constant Wave Length-Constant Angular Velocity) scheme of recording data at an almost constant wave length by changing the recording clock frequency in accordance with a track position while keeping the rotational speed of a disk constant.
In disk recording/reproducing apparatuses of the CAV and CLV schemes, since the bit rate of a reproduced signal is almost constant, and the maximum frequency of the reproduced signal hardly changes, a reproduced signal equalizer having fixed equalization characteristics is used.
FIG. 3 shows a conventional disk-reproduced signal equalizer applied to a CAV scheme disk recording/reproducing apparatus. An optical head 13 of the disk recording/reproducing apparatus reads an analog reproduced signal S11 of a constant bit rate from an optical disk 11 which is rotated at a constant angular velocity by a disk motor 12. In a disk-reproduced signal equalizer 14, the analog reproduced signal S11 is amplified by a buffer amplifier 141 and is delayed by delay circuits 142 and 143, each having a delay time .tau.. The analog reproduced signal S11 is then input to a differential amplifier circuit 144 via buffer amplifiers 145 to 147 and tap gain setting resistors 148 to 150 so as to be output as an equalized reproduced signal S12 which has undergone predetermined equalization processing. Letting fp be the emphasized peak frequency of the analog reproduced signal S11, fp=1/2.tau..
FIG. 4 shows another conventional disk-reproduced signal equalizer. In a disk-reproduced signal equalizer 15, an analog reproduced signal S11 from an optical head 13 is amplified by a buffer amplifier 91 and is digitized by an A-D converter 92. In this case, A-D conversion is executed by using a clock signal S13 having a constant period, which is supplied from a clock generator 98. The digitized reproduced signal is subjected to predetermined equalization processing in a known transversal filter 97. More specifically, delay circuits 94a to 94d operate in accordance with the clock signal S13 to delay the digitized reproduced signal by a predetermined time and output the respective tap outputs to coefficient multipliers 95a to 95e. An adder 96 adds the tap outputs respectively multiplied by coefficients by the coefficient multipliers 95a to 95e, and outputs an equalized reproduced signal S22.
In a CWL-CAV scheme disk recording/reproducing apparatus, the bit rate increases as a head moves toward the outer peripheral side of a disk, so that the maximum frequency which allows a recording/reproducing operation increases. For this reason, if the ratio of the peripheral velocity on the outer peripheral track of a disk to that on the inner peripheral track is large, proper equalization processing is difficult to perform. That is, even if the equalization characteristics are optimized on the basis of the maximum frequency of a reproduced signal on the inner peripheral track, since the maximum frequency of the reproduced signal on the outer peripheral track increases, predetermined equalization processing cannot be performed.